1. Field of the Invention
The present invention relates to an injection molded article of a resin. The present invention relates more particularly to an injection molded article which can be applied to an optical component such as a lens barrel or a finder in an optical device such as a camera or a microscope.
2. Description of the Related Art
Many of optical components such as a lens barrel and a finder of a camera are molded of a resin by injection molding. On a surface facing an optical axis such as an inner surface of the injection molded optical component, in many cases, light intercepting ridges for preventing reflection of light on the surface are formed.
As shown in FIG. 18, light intercepting ridges 1 are formed in such a manner as to repeat wavy undulations periodically on one surface of an optical component 2. In a camera component, the ridges are formed in such a manner that, for example, a pitch is about 1 mm long and amplitude is about 0.3 mm height.
When undulations such as the light intercepting ridges are formed at an injection molding time, there are sometimes generated a deformation such as warpage attributable to the undulation in a molded article. FIGS. 19 and 20 are explanatory views of causes for the warpage. FIGS. 19 and 20 show on a left side the inside of a cavity immediately before the article is removed from a mold after the injection molding. FIGS. 19 and 20 show on a right side a temperature distribution graph of the resin in the cavity, the ordinate shows a position in a plate thickness direction along which the undulation is formed, and the abscissa shows a temperature. FIG. 19 shows a case where the light intercepting ridges 1 are deep, and FIG. 20 shows a case where the light intercepting ridges 1 are shallow.
In a case where the light intercepting ridges 1 are deep as shown in FIG. 19, an area of a resin 3 brought into contact with a mold 4 is large in the vicinity of the light intercepting ridge 1, and therefore a resin temperature of a contact portion drops. On the other hand, since the resin in the vicinity of the light intercepting ridge 1 at a low temperature functions as an insulating material, heat of the resin of the portion slightly inward the light intercepting ridge 1 is not easily conducted to the mold 4. As a result, in the graph, a temperature curve is asymmetrical to a temperature peak 5. As shown on the right side of FIG. 19, a portion of the molded article in the vicinity of the light intercepting ridge 1 is set at a temperature which is slightly higher than that of a central portion of the cavity. Therefore, a molding shrinkage in the vicinity of the light intercepting ridge 1 is larger than that in the vicinity of the side opposite to the light intercepting ridge 1, and an upward warpage is generated in the molded article as shown by an arrow A.
In a case where the light intercepting ridge 1 is shallow as shown in FIG. 20, the area of the resin 3 brought into contact with the mold 4 is large in the vicinity of the light intercepting ridge 1 in the same manner as in FIG. 19. However, since the light intercepting ridge 1 is shallow, the resin functioning as the insulating material at the low temperature has a small amount, and is distributed in periphery of the cavity as compared with a situation of FIG. 19. Therefore, the heat of the resin of the portion slightly inward from the light intercepting ridge 1 is less prevented from being conducted to the mold. Therefore, the heat of the resin is more easily conducted to the mold 4 on a light intercepting ridge 1 side of the molded article as compared with the opposite side. As shown on the right side of FIG. 20, the temperature of the portion closer to the light intercepting ridge 1 than the central portion of the cavity is slightly lower than the previous case. Therefore, the molding shrinkage in the vicinity of the side opposite to the light intercepting ridge 1 is larger than that in the vicinity of the light intercepting ridge 1, and a downward warpage is generated in the molded article as shown by an arrow B.
To cope with the above-described warpage, in Japanese Patent Application Laid-Open No. 2002-36309, a temperature difference is made between a fixed mold and a movable mold to thereby control a distribution of a resin temperature. In Japanese Patent Application Laid-Open No. 6-55597, the resin temperature is controlled between the vicinity of a gate and a flow terminal end portion of the resin.